The present invention relates to hard contact lenses having an excellent oxygen permeability.
Contact lenses put presently on the market are classified into two large groups, i.e. soft contact lenses made of a hydrophilic polymer having a 2-hydroxyethyl methacrylate as a main component or a soft hydrophobic polymer such as silicone rubber, and hard contact lenses made of a hard polymer such as polymethyl methacrylate. The hard contact lenses are generally inferior in wearing sensation to the soft contact lenses, but they have excellent visual power correcting effect and durability and also have the characteristic advantages in the hard contact lenses such as easiness in handling, and accordingly they are widely used at present.
Hard contact lenses made of a polymethyl methacrylate have the fatal defect that it is difficult to supply oxygen required in metabolism of corneal tissue (the iris of the eye) from the atmosphere to cornea through the lens materials, in other words, the oxygen permeability is bad. Consequently, wearing of the hard contact lenses for a long period of time causes metabolic trouble of the corneal tissue.
In recent years, however, the above problem has been dissolved to some extent by appearance of an oxygen permeable hard contact lens made of a copolymer of methyl methacrylate and a special methacrylate compound having a silyl or siloxanyl group (Si--O bond) in its molecule (hereinafter referred to as "silicone-containing methacrylate"), and the hard contact lenses have rised in clinical estimation.
However, the proposed copolymer of the methacrylate compound having a silicone is inferior in hardness and rigidity to polymethyl methacrylate used as a material of usual hard contact lenses, and also is fragile. Lack of hardness and rigidity makes it easy to take scratches on the lens surface, bad in durability and makes it difficult to prepare contact lenses of a constant quality according to a predetermined lens contour.
Accordingly, in order to obtain oxygen permeable hard contact lenses having hardness and rigidity desired for hard contact lenses, the proportion of methyl methacrylate must be increased while decreasing the proportion of the above-mentioned silicone-containing methacrylate compound, which has relatively low hardness. But, this results in falling into the dilemma that the oxygen permeability of the obtained copolymer is decreased.
Besides, it is preferable to use the materials having the high refractive index for improving the wearing sensation by making the contact lens thinner.